High Strength Linepipe With Excellent HAZ Toughness

Abstract

The API 5L-X65 steel plates for low temperature service were produced using the thermo-mechanical control process (TMCP) with the optimum micro-alloying addition. Featuring of the additions are as low amount of titanium, calcium, niobium, and vanadium as possible, for high heat affected zone (HAZ) toughness and strength. Controlling titanium and nitrogen and the Ti/N ratio, a large number of TiN dispersed finely are formed in steel and the austenite grain size near a weld fusion line is refined remarkably owing to strong pinning effect of TiN. Calcium addition promotes ferrite nucleation, so that increase in fine polygonal ferrites makes microstructure of HAZ much finer. Niobium and vanadium content are reduced, because carbide precipitates are formed when the coarse grain HAZ is reheated around 700 degree C and the precipitation hardening deteriorates HAZ toughness. The trial manufacturing of the 19.5mm, 26.9mm and 31.4mm thick X65 grade UOE pipes was finalized with the satisfactory results. The toughness of longitudinal submerged-arc welds was more than 50 J in Charpy V-notch impact test at −30°C.

Role of Ti and N in line pipe steel welds

“…41 Jiao et al 42 stated that the best toughness achieved at Ti/N53 in high Nb pipeline steels and recommended the industry control of Ti/N in the range of 2-3. Hillenbrand et al 41 11 Effect of Ti/N ratio on grain size and toughness 40 Zhu et al further claimed that Ti content of the steel must be ,0?025% to avoid the detrimental effects on the HAZ toughness of the longitudinal weld seam. The above review advised that different Ti, N additions as well as Ti/N ratios were considered to be optimal for HAZ properties, especially toughness.…”

“…As shown in Fig. 11, recent work from Okastu et al 40 investigated X65 pipeline steels having Ti/N ratio in the range of 2–4. It appears that higher Ti/N ratio can lead to deteriorations in HAZ toughness and best CGHAZ toughness achieved at Ti/N = 2.…”

Titanium microalloyed steels

“…However, with an hypostoichiometric Ti addition (Ti/N=2), He and Baker 205 found that with a low finishing rolling temperature,800°C, excellent toughness and mechanical properties were obtained due to grain refinement and strain induced precipitation. Using the Ti/N ratio as a guide to avoiding poor toughness was also supported by the work of Okastu et al, 321…”

“…However, with an hypostoichiometric Ti addition (Ti/N = 2), He and Baker [205] found that with a low finishing rolling temperature, 800°C, excellent toughness and mechanical properties were obtained due to grain refinement and SIP. Using the Ti/N ratio as a guide to avoiding poor toughness was also supported by the work of Okastu et al [321] who investigated X65 steels with Ti/N ratios from 2 to 4. Although most investigations have considered the Ti/N ratio as a reasonable guide for obtaining MA steels with acceptable toughness levels, Shen and Hansen [322] concluded from their work, that any effects related to coarse TiN particles should be analysed using [Ti][N] product rather than Ti/N ratio.…”

Acicular Ferrite Identification in Heat‐Affected Zone of a Ti‐Killed High Strength Low Alloy Steel

“…[3] To enhance the toughness of HAZ, various techniques have been developed, relying on restricting austenite coarsening, stimulating intragranular nucleation, and reducing M/A constituent sizes. [4][5][6] Among them, developing an acicular ferrite (AF) dominant microstructure has been regarded as an effective measure, and frequently AF has been known as the optimal microstructure in steel HAZ and welds, because of its superior combination of strength and toughness. [7,8] AF was first found in steel welds as laths that nucleate heterogeneously on small nonmetallic inclusions and grow in many different directions from these point nucleation sites.…”